WASHINGTON – U.N. weapons inspectors went back to Baghdad for the first time in four years yesterday, on a mission that could lead to a new Gulf War – as Iraq upped the ante by firing once again on U.S. and British warplanes over a “no-fly” zone.

The White House says the Iraqis are in “material breach” of U.N. resolutions simply by firing on the planes – in other words, that Iraq is already asking for war.

Defense Secretary Donald Rumsfeld termed the latest Iraqi air attacks, an “unacceptable” provocation. Allied planes struck back by pounding two Iraqi air defense communications southeast of Baghdad as well as targets in northern Iraq.

The no-fly zones were set up to protect Iraqi opponents of Saddam after the 1991 Gulf War, and Baghdad’s stepped-up attacks on allied planes suggest Saddam may be testing U.S. and U.N. resolve as the tough new inspections start.

“Make no mistake about it – our aircraft will continue to respond accordingly,” said White House spokesman Scott McClellan, who contended the attacks violated the Security Council resolution setting up the new weapons inspection regime.

That resolution bars Iraq from interfering with anyone enforcing U.N. policies, but so far, the Bush administration hasn’t taken the issue of the Iraqi attacks to the Security Council, where some members question the U.S. view on that point.

Bush has vowed “zero tolerance” of Iraqi defiance and delays. But McClellan didn’t directly respond to a question of whether the president would prefer to wait and make an issue of Iraqi defiance directly related to the weapons inspections.

There’s no formal link between the air patrols and the tougher U.N. weapons inspections, but analysts say it’s clear that the allied warplanes are trying to destroy Iraqi defenses in case it does come to war.

Chief U.N. weapons inspector Hans Blix and nuclear inspector Mohamed ElBaradei landed in Iraq on a chartered C-130 cargo plane yesterday with an advance team of about 30.

“The situation is tense at the moment,” Blix conceded as he began what President Bush says is Iraq’s last chance to give up its weapons of mass death.

The showdown on weapons inspections could come in just a few weeks, since they’re slated to resume Nov. 27.

Iraq must submit a full list of all chemical, biological and nuclear programs by Dec. 8 – but Iraq insists it has none at all.

The inspectors – who vow to check as many as 700 suspect sites – are slated to report back by Jan. 26.

Bush has said the United States could lead its own war coalition against Saddam without the world body.

The 300 weapons inspectors from 48 countries – who could include, by Blix’s own admission, some spies ready to tip off Saddam – will be armed with high-tech gear that can detect the tiniest trace of anthrax or nuclear-weapons material.

SNIFFING OUT SADDAM’S CACHE

A look at how United Nations inspectors will work in Iraq:

1. Arrive at a suspected weapons site by car or helicopter, usually in small teams, depending upon the site.

2. Request everyone stop working but remain on site for interviews. A U.N. helicopter patrols the area to ensure no one leaves with evidence.

3. Examine the facility. Inspectors have the right to remove or destroy any equipment, materials, or documents used to produce weapons of mass destruction.

4. Clear the stockpile. Any facility determined to be part of Iraq’s weapons program is destroyed with the approval of the United Nations.

5. Monitor. Keep a close watch on all of Iraq’s “dual purpose” facilities with surveillance cameras, continued testing of samples and satellite surveillance.

* Surprise inspections were difficult to accomplish. Iraq required 15 hours advance notice for any U.S. flight. This “stand down” time gave the Iraqis ample opportunity to anticipate where the inspectors would go, and conceal or destroy evidence.

THE DISCOVERY PHASES

Analyze satellite imagery and aerial photography for changes in construction sites, power lines, ground cover (new tree lines, recent soil disruptions, etc.) and evidence of security changes. Inspectors already have a list of about 800 sites they want to visit.

Review import/export documents for all materials entering Iraq that can be used in weapons production.

Set up shop in the Baghdad office, vacant since 1998, as well as two additional locations, in Mosul and Basra. The chemical labs in the Baghdad office will be moved to a separate, more secure location.